(1) Field of the Invention
The present invention relates to an electrostatic chuck.
(2) Related Art Statement
At present, electrostatic chucks are used for attracting and holding semiconductor wafers in conveying, film-forming processes such as light exposure, CVD and sputtering, fine machining, washing, etching, dicing, etc. for the semiconductor wafers. In JP-B 5-87177, a laminate is produced in a filmy thickness of 30 to 400 .mu.m by successively laminating a first insulating layer, a first bonding layer, an electrode layer, a second bonding layer and a second insulating layer, and an electrostatic chuck is produced by bonding this laminate to a metallic substrate. The first insulating layer, which is arranged between the electrode layer and an object to be treated, preferably has a thickness of 5 .mu.m to 75 .mu.m, which is tried to be made as small as possible so long as the insulating layer can withstand voltage applied. This meets a theory that the smaller the thickness of the insulating dielectric layer of the electrostatic chuck, the greater is the attracting force.
Turning to this point in more detail, the electrostatic attracting force (Coulomb's force) is in inverse proportion to the square of a distance between objects upon which this force acts. As the insulating dielectric layer of the electrostatic chuck becomes thicker, the distance between the electrode and the object to be treated proportionally increases. Correspondingly, the electrostatic attracting force decreases in inverse proportion to the square of the thickness of the insulating dielectric layer. For this reason, it is necessary that the insulating layer is made as thin as possible so as to increase the electrostatic attracting force.
In JP-A 2-160444, two or more laminate layers each constituted by an electrode and an insulating layer are formed on a substrate. The insulation resistances of the insulating films are made different from each other so that voltage to be applied to each electrode may be selectively controlled. This publication describes that the thickness of each insulating film is appropriately around 300 .mu.m. For, in order to increase the electrostatically attracting force, the insulating film needs be thinner as mentioned above, whereas in order to prevent dielectric breakdown under application of high voltage, a certain thickness is necessary. To meet both of these contradictory requirements, the thickness of a few tens .mu.m to 300 .mu.m was appropriate. As described in JP-A 2-160444, as the temperature rises, the volume resistivity of the insulating film decreases. Accordingly, as the temperature rises, the leakage current increases in the insulating film, so that the semiconductor film already formed on the the semiconductor wafer is unfavorably broken.
Further, Japanese Utility Model Application Laid-open No. 2-120831 discloses that grooves are formed on a semiconductor wafer-placing face and helium gas is fed into the grooves. That is, a substrate to be treated, such as a semiconductor wafer, need be heated or cooled depending upon the purpose of a process employed. For this reason, it is necessary that a heating source or a cooling source is installed under the substrate of the electrostatic chuck and heat is exchanged between the substrate and the semiconductor wafer or the like. At that time, since the semiconductor wafer merely contacts the attracting surface of the electrostatic chuck, so that they are placed in an adiabatic vacuum state inside a vacuum chamber of a semiconductor-producing apparatus. That is, since no heat conduction occurs through convection, heat conduction is very small. Thus, as mentioned above, the grooves are filled with helium gas so that heat may be effectively conducted between the semiconductor wafer and the attracting surface through the helium gas.
When a semiconductor wafer is treated in the state that it is attracted upon an electrostatic chuck, such an electrostatic chuck is used over a wide temperature range. As mentioned above, if the thickness of an insulating film of the electrostatic chuck is about a few .mu.m to 300 .mu.m, for example, a current leaked from the insulating film largely increases at more than 300.degree. C., even though extremely large attracting force may be obtained at room temperature. Consequently, it was made clear that a semiconductor film already formed on the semiconductor wafer might be broken. For this reason, a special construction as described in JP-A 2-160444 needed to be employed so that the electrostatic chuck might be used in a high temperature range. However, such a construction is extremely complex, and it does not offer a direct solution against the above problems.
It may be considered that a material maintaining a high volume resistivity even at high temperatures is selected or developed. However, a plastic material having a high volume resistivity generally possess low heat resistance, and it is essentially difficult to use such a plastic material in a high temperature range. On the other hand, many of ceramic materials having high heat resistance possess their volume resistivity which decrease in a high temperature range. In addition to the requirement for the volume resistivity, the substrate of the electrostatic chuck must satisfy other requirements such as the mechanical strength, but it is generally difficult to select or develop a material satisfying the above requirements. Japanese Utility Model Registration Application Laid-open No. 2-120,831 also suffer the above problems.
In view of the above, the present inventors produced insulating dielectric layers having thicknesses of a few tens .mu.m to 300 .mu.m from various ceramic materials, and examined them with respect to attracting force and leakage current. In general, in order to exhibit sufficiently high attracting force, the insulating dielectric layer needs to have a volume resistivity of 1.times.10.sup.13 .OMEGA..multidot.cm or less in an operating temperature range.
It was clarified that an electrostatic chuck having an insulating dielectric layer with a volume resistivity, for example, in a range of 1.times.10.sup.11 to 1.times.10.sup.13 .OMEGA..multidot.cm at room temperature exhibited high attracting force in a range of room temperature to 200.degree. C., but leakage current largely increased at temperatures of more than 200.degree. C., which might damage a semiconductor wafer. It was also clarified that the electrostatic chuck having the insulating dielectric layer with the volume resistivity of 1.times.10.sup.14 .OMEGA..multidot.cm to 1.times.10.sup.16 .OMEGA..multidot.cm at room temperature had a high attracting force in a temperature range of 100.degree. C. to 500.degree. C., but its leakage current largely increased when the temperature was more than 500.degree. C. so that the semiconductor wafer might be damaged. It was further clarified that in the electrostatic chuck with the insulating dielectric layer having the volume resistivity of 1.times.10.sub.9 .OMEGA..multidot.cm to 1.times.10.sup.10 .OMEGA..multidot.cm at room temperature exhibited high attracting force in a temperature range of -20.degree. C. to 100.degree. C., but it damaged the semiconductor wafer due to largely increased leakage current at temperatures of more than 100.degree. C.
In this way, it was clarified that although the conventional ceramic electrostatic chucks all exhibited sufficiently high attracting forces in an optimum temperature range, the leakage currents largely increased if the operating temperature rose and the volume resistivity of the insulating dielectric ceramic layer decreased to 10.sup.9 .OMEGA..multidot.cm or less. Therefore, it was clarified that the conventional electrostatic chucks had a problem in such a use in which a use temperature range is wide, for example, in such a case where various treatments are effected for semiconductor wafers chucked.
Further, in Japanese Utility Model Registration Application Laid-open No. 2-120831, heat needs to be conducted between the semiconductor wafer and the electrostatic chuck uniformly as viewed planarly from the attracting surface thereof. For, even if the temperature of the attracting surface of the electrostatic chuck is equal, a large difference in temperature of the surface of the wafer occurs between a helium gas-filled portion and a helium gas non-filled portion inside the grooves. Consequently, the quality of the resulting semiconductor film varies, which may cause unacceptable products during the production process. Therefore, it is necessary to keep the pressure of the helium gas constant in every portion inside the grooves.
However, in the locations of the actual attracting chuck from which helium gas is to be fed are limited, and their feed openings of the helium gas-feeding locations are away from adjacent ones. Therefore, as the location goes away from a blow-out opening of the helium gas, the pressure of the gas rapidly decreases. In particular, as mentioned above, the thickness of the insulating dielectric layer is merely around a dozen .mu.m to 300 .mu.m, and the insulating dielectric layer merely has a minimum thickness required to maintain a necessary dielectric breakdown strength. This dielectric breakdown strength is a value of a minimum thickness portion of the insulating dielectric layer. For these reasons, the thickness of the grooves must inevitably be set at a few .mu.m to a dozen .mu.m. However, the grooves having a depth of a few .mu.m to a dozen .mu.m gives a large resistance against diffusion of the gas, so that the gas is not sufficiently diffused. Consequently, a large pressure difference occurs inside the grooves and the temperature varies in the semiconductor wafer, so that the quality of the film formed becomes non-uniform. Simultaneously with this, increase in the depth of the grooves causes antonymy that dielectric breakdown may occur between the grooves and the electrode.